Control of a jet engine is accomplished by providing a known mass flow of fuel based on a demand which may be established by an electronic control. The electronic control may be digital microprocessor based or may be analog and constructed of discrete electronic components. Accurate and continuous control of mass fuel flow is required over a broad range of engine operating conditions in order to provide reliable, fuel efficient operation. Some mass flow meters may not provide an accurate indication of mass flow while engine conditions are changing or fuel flow is low.
Several seconds of constant fuel flow at a flow rate above 20% of the maximum rated fuel flow is normally required for a mass flow meter to generate a mass flow signal accurate enough for engine control use. Therefore, a volumetric fuel flow methodology is generally used instead. For example, a volumetric fuel flow methodology may be implemented by controlling the flow area of a metering valve while maintaining a known differential pressure across the metering valve. The flow area is the size of the metering valve aperture which can be measured and controlled by external means.
The volumetric flow method in general use assumes that the mass density of the fuel remains constant during operation. A preselected value of mass density is stored in the electronic control circuitry. The electronic control circuitry determines the desired volumetric flow by dividing the desired mass flow by the preselected mass density. However, discrepancies may occur between the actual mass density and the preselected mass density when fuel type changes or when the fuel temperature varies. Engine designers may compensate for these discrepancies by designing in increased tolerances in the rotating parts of the gas turbine engine, or by attempting to make corrections that simulate changes in the mass density, both of these techniques tend to increase fuel consumption or prolong engine transients.